It is called "Intragovermental Holdings." It contains the HI, SS, and federal trust funds. For example, the SSTF contains $2.7 trillion in non-market interest bearing T-bills. How did the SSTF get the T-bills?
When SS was getting more in revenue that it paid out in benefits, the "surplus" was deposited into the General Fund. In return, Treasury issued T-bills in the amount of the surplus and deposited them into the SSTF. The SSTF represents the good faith and credit of the USG to honor those T-bills in much the same way it honors the T-bills held by the Chinese. Since SS has been running in the red since 2010, i.e., more is being paid out in benefits than in revenue received, some of those T-bills are being cashed in to make up the shortfall.
Intragovernmental Holdings are obligations by the USG, which has borrowed money from SS, HI, and federal pension funds, not the other way around.
I stand corrected.
A very clear explanation.